The present invention relates to an electrical insulating material which is superior in heat resistance, mechanical properties, film flatness, handling qualities, and electrical characteristics. More particularly, the present invention relates to an electrical insulating material which comprises biaxially oriented polyethylene naphthalate film having a center line average roughness of 0.008 to 0.08 .mu.m as measured on the surface thereof and a resistivity of 5.times.10.sup.7 to 5.times.10.sup.10 .OMEGA..multidot.cm as measured in the molten state.
Polyethylene terephthalate film has hitherto been in general used as an electrical insulating material because of its outstanding heat resistance, mechanical and electrical properties, and processability. However, the conventional polyethylene terephthalate (Class E: a continuous allowable temperature of 120.degree. C.) is insufficient in heat resistance in the case where the polyethylene terephthalate is to be used in recent electrical and electronic machines and equipment which have been miniaturized and lightened weight, and have high efficiency. Thus there is a demand for a new electrical insulating material having excellent heat resistance.
Use embodiments of the electrical insulating material are set forth below.
(1) Electrical insulating parts comprise the electrical insulating material and an electrical insulating paper such as kraft paper, etc. stuck to the electrical insulating material.
The electrical insulating parts are used as insulating parts of an electrical machinery and apparatus, and particularly, are used as insulating parts of motor, transformer, etc. of a heavy electric apparatus. Namely, the electrical insulating parts are used as insulating parts of a slot insulation, a layer insulation and a coil-end insulation of an electric motor and a generator. Also, the electrical insulating parts are used as insulating parts of a coil-to-coil insulation, a turn-to-turn insulation, an insulation of a high-tension circuit such as a lead wire of the transformer.
(2) An electric wire comprises a conductor and the electrical insulating material coated on the conductor.
The electrical insulating material is used for insulating the conductor and cable. Also, the electrical insulating material is used as a primary coating material or a secondary coating material of the electric wire.
(3) A flexible printed circuit board comprises the electrical insulating material and a copper foil or a ribbon-like copper core stuck on the electrical insulating material.
The flexible printed circuit board is superior in the freedom degree of an electric circuit such as curvilinear circuit, collapsible circuit etc. to a conventional rigid print board.
Polyethylene naphthalate film is a comparatively inexpensive electrical insulating material (Class F: a continuous allowable temperature of 155.degree. C.) which meets the above-mentioned requirements. The characteristic properties thereof are described in Japanese Patent Publication Nos. 53-35280 (1978) and 54-1920 (1979), and Japanese Patent Laying-Open Nos. 48-43198 (1973), 48-43200 (1973), 48-53299 (1973), 49-132600 (1974), 49-32200 (1974), and 50-133279 (1975).
It is generally known that polyethylene naphthalate film is basically superior in heat resistance, and mechanical and electrical properties. However, the conventional polyethylene naphthalate has a shortcoming of becoming poor in film flatness and handling qualities when the film thickness is reduced. In addition, it is poor in electrical properties, especially volume resistivity at high temperatures, e.g. 120.degree. C. So far, very few efforts have been made to improve these characteristic properties. Therefore, the improvement of these characteristic properties is expected, because that polyethylene naphthalate film can be used as a good electrical insulating material if these characteristic properties is improved.
With the foregoing in mind, as a result of the present inventors' extensive studies on polyethylene naphthalate film, it has been found that polyethylene naphthalate film is highly improved in film flatness, handling qualities, and electrical properties if it has a surface roughness in a certain range and it also has a resistivity in a certain range as measured in the molten state, and on the basis of this finding the present invention has been attained.